1. Field of the Invention
The present invention relates to an optical head device in an optical disk drive which can carry out recording/reproduction of information into/from an optical recording media (optical disks) in a plurality of standards having different operation wavelengths, i.e., at least some of CDs (compact disks), DVDs (digital versatile disks), S-DVDs, disks applicable for a blue operation wavelength and so forth, with an employment of a polarization diffraction grating or a polarization hologram device for a beam splitting purpose therein.
2. Description of the Related Art
An optical system which splits a beam with a diffraction device and applies a reflected light from an optical disk which is an optical information recording medium onto a light detection device is provided in an optical head device (optical pickup) of an optical disk drive in various types. As the above-mentioned diffraction device, a polarization diffraction device is known, for example.
For example, a usage of such a diffraction device which has an optical anisotropy formed of an optical anisotropic polymer as a diffraction device in an optical head device which performs recording/reading of information by applying a beam from a light source through the diffraction device onto an optical recording medium is disclosed by Japanese laid-open patent application No. 9-50642, entitled “Optical Head Device and Its Manufacture Method”, for example.
Such an example of a conventional diffraction device is shown in FIG. 1. A medium 102 showing a birefringence (optical anisotropy) with a rectangular patterned indented surface 102a formed on a transparent substrate 101 is arranged. Thereon, a medium 103 having an optical isotropy is applied, and, after that, a transparent substrate 101′ is put thereon. Thus, a polarization diffraction grating 107 is produced. Therein, a diffraction grating which shows a polarization property (optical anisotropy) is obtained by making the refractive index of the isotropic medium 103 equal either to an ordinary-ray refractive index or an extraordinary-ray refractive index of the birefringent medium 102. Thereby, the characteristics can be provided therein in that approximately all the beams having a certain polarization direction is transmitted thereby while approximately all the beams having a polarization direction perpendicular thereto is diffracted thereby.
When such a polarization diffraction grating 107 is used as a beam splitting device in an optical head device of an optical disk drive, a setting is made such that a going beam directed toward an optical recording medium or an optical disk from a light source is completely transmitted by the polarization diffraction grating 107 so that the beam is efficiently applied to the optical recording medium. After that, a reflected beam from the optical recording medium is returned to the polarization diffraction grating 107 after the polarization direction is made perpendicular through a ¼-wavelength plate disposed in the beam path so that the returning beam is completely diffracted by the polarization grating 107 into a light detection device with high light-usage efficiency. Thus, it becomes possible to realize an efficient optical head device in which the light-usage efficiency is high either on the going beam or on the returning beam.
In case the above polarization diffraction grating is disposed nearer to the light source part so as to miniaturize a space needed around the light source and light detecting device, it is necessary to make the pitch in the polarization diffraction grating smaller as possible so as to increase the diffraction angle on the returning beam.
However, when the pitch is made smaller, one problem may occur. This is a problem concerning an angle-dependency of the diffraction efficiency. FIG. 2 shows a relation between the incidence angle onto the polarization diffraction grating and the +1-th diffraction efficiency. In FIG. 2, a curve 201 shows the characteristic of a grating with a comparatively large grating pitch (more than 4 micrometers). When the grating pitch is thus relatively greater, it acts as a thin-plane-type diffraction grating, the 1-th incidence-angle-dependent diffraction efficiency has a quite flat characteristic as shown, the diffraction efficiency is approximately 40% and the diffraction efficiency hardly changes with a change in the incidence angle.
On the other hand, the characteristic when the grating pitch is relatively small is shown as a curve 202. The curve 202 shows the incidence-angle-dependent +1-th diffraction efficiency characteristic in case the grating pitch is set as 1.6 micrometers. As shown, when the grating pitch is made smaller, the grating type thereof changes from a thin plane-type diffraction grating into a thick volume-type diffraction grating. The characteristic in this case is such that the diffraction efficiency at a specific incidence angle θB has a peak with respect to the diffraction efficiency at 0 degree of incidence angles as shown.
A Q value of a diffraction grating is defined as a criterion for distinguishing the above-mentioned thin grating and thick grating. Where the operation wavelength is λ, the grating thickness is T, the grating average refractive index is ‘n’, and the grating pitch is ‘d’, the Q value of the diffraction grating is expressed by the following formula:Q=2πλT/nd2Then, for example, upon Q<1, it is distinguished as a thin plane-type grating, while, upon Q>10, it is distinguished as a thick volume-type grating. Upon 1<Q<10, it is distinguished as a grating in an intermediate range between a plane type and a volume type.
In the example shown in FIG. 2, assuming that the operation wavelength λ=0.66 micrometers, Q=0.64 and it is distinguished as a plane-type grating for the curve 201, while for the curve 202, Q=4.0 is obtained, thus, it somewhat shows the characteristic of a volume-type grating, and it is distinguished as a grating in an intermediate range between a plane-type grating and a volume-type grating. That is, the diffraction efficiency has a peak for a specific incidence angle θB as mentioned above. This specific incidence angle θB is called a Bragg angle, and it is expressed as follows:θB=sin−1(λ/2d)As the grating pitch is 1.6 micrometers in the case of the curve 202 of FIG. 2, θB=11.9 degrees is obtained assuming that the operation wavelength of λ=0.66 micrometers. That is, the diffraction efficiency is highest when the incidence angle is 11.9 degrees in the air, and, thus, higher than that in the case of right-angle or perpendicular incidence. In case of the curve 202 of FIG. 2, a maximum of 70% or more of diffraction efficiency is obtained there.
When the pitch of the diffraction grating is made smaller and thus the polarization diffraction grating having the characteristic of the above-mentioned volume-type grating is used in an optical head device so as to dispose the polarization diffraction grating nearer to a light source part and thus miniaturize an optical system needed around the light source and an light detection device, a problem may arise.
The problem will now be discussed with reference to a case of applying the polarization diffraction grating which has the pitch made smaller into an optical head device with a configuration shown in FIG. 3. As shown, the optical head device includes a light source 108 is made of a semiconductor laser, or so, a light detection device 107 including a light-receiving-surface-divided photodiode, a ¼-wavelength plate 111, a collimator lens 110, a polarization diffraction grating 107, and an object lens 112 for focusing an incident beam onto an optical recording medium 113.
A beam emitted from the light source 108 is previously set such that it is approximately completely transmitted by the polarization diffraction grating 107. Then, after being collimated by the collimator lens 110, the beam turns into a circle polarization with the ¼-wavelength plate 111, and it is focused onto the optical recording medium 113 with the object lens 112. The reflected light from the optical recording medium 113 is then transformed in its polarization direction such that it intersects perpendicularly with that in the going beam through the ¼-wavelength plate 111, turns into a convergence beam by the collimator lens 110, and thus, is applied to the polarization diffraction grating 107.
Since this beam has the polarization which intersects perpendicularly to that in the going beam as mentioned above, this beam is approximately completely diffracted thereby, thus a +1-th diffracted light thereof is applied to the light detection device 109, and there, predetermined signals are detected therefrom by the light detection device 109. Assuming that the direction of tracks of the optical recording medium 113 is perpendicular to the figure, a push-pull signal as a tracking servo signal is acquired from the signal expressing a difference in luminous energy between both sides of the light spot formed on the light detection device 109 about the optical axis of the beam thus having returned from the optical recording medium.
When the diffraction grating of the curve 201 shown in FIG. 2 having the comparatively large grating pitch is applied in this system, as the diffraction efficiency is symmetrical with respect to +/− angle variation about the central point at which the incidence angle is 0 degrees, the push-pull signal obtained from the diffracted light indicates a true tracking servo signal. However, when the diffraction grating of the curve 202 shown in FIG. 2 having the comparatively small grating pitch is applied, as the diffraction efficiency is not symmetrical with respect to +/− angle variation about the central point at which the incidence angle is 0 degrees, the push-pull signal obtained from the thus-diffracted light does indicate a true tracking-servo signal. In fact, as shown in FIG. 2, on the curve 202, the diffraction efficiency increases as the angle increases in the plus direction while the diffraction efficiency decreases as the angle increases in the minus direction, in the range A shown in FIG. 2 showing an actual range of incident angle in a practical optical head device for example.
Such an imbalance in the diffraction efficiency between both sides causes an offset in the push-pull signal. Thereby, even when the optical head is positioned accurately on a track, the tracking servo signal does not indicate a zero value, and, thus, a proper tracking servo control may not be achieved in the optical disk drive.
On the other hand, improvement in the speed of reproduction is demanded for such an optical disk drive carrying such an optical head device. In order to raise the S/N ratio in signal detection for the purpose of improvement in the speed of reproduction, it is required that, as for the polarization diffraction grating used in the optical head, a +1-th diffracted light should have a high diffraction efficiency therein on an occasion of an incidence thereonto at near the right angle (approximately 0±5 degrees). However, when the grating pitch is made smaller as mentioned above, the diffraction efficiency has a peak at a specific incidence angle (Bragg angle) other than the right angle, and thus, the diffraction efficiency near the right-angle incidence may be degraded relatively.
For the purpose of miniaturizing in size and reducing the costs of such an optical head device or an optical pickup, an optical system employing a polarization hologram device as a polarization beam-splitting device takes attention. Same as the above-mentioned polarization diffraction grating, the polarization hologram device is applied for the purpose of separating a going beam and a returning beam. Such a type of beam splitting device is advantageous in terms of the size thereof in comparison to a conventional polarization beam splitter or so.
Furthermore, the polarization hologram device has other advantages in that the beam path design on the optical system becomes easier, and also, the number of parts/components can be reduced, since a signal detection device can be disposed on a same plane on which a laser light source is disposed. Moreover, by applying the polarization hologram device, a provision of a single common beam path is enough even in case writing/reading is performed on a plurality of recording media with different recording densities, such as a CD, a DVD, and an optical disk suitable for a blue wavelength, for example.
As such a polarization hologram device, Japanese laid-open patent application No. 2000-221325 discloses a technology of manufacturing a polarization beam-splitting device by which cyclic grating is formed by performing a patterning exposure of a polydiacetylene orientation film formed on an optical isotropic substrate with an ultraviolet ray at a sufficient yield, for example. According to this, in case the patterning exposure of the polydiacetylene orientation film acting as a birefringent material layer is performed with the ultraviolet ray in manufacture of the polarization beam-splitting device, the cyclic grating parallel to the orientation direction is formed by a way of making coincident the orientation direction of polydiacetylene orientation film with the patterning direction. When the cyclic grating is thus made in coincidence with the above-mentioned orientation direction, the diffraction efficiency can be increased thereby, and, also, variation in the diffraction efficiency can also be well controlled.
Moreover, Japanese laid-open patent application No. 2000-75130 discloses an inexpensive polarization beam-splitting device, for which production thereof does not take a much time, and, also, it does not need a complicated production process. As to this device, in order to separate two polarization components which intersect perpendicularly, a birefringent film having a refractive index variable according to a polarization plane of an incident light is loaded onto a transparent substrate as a cyclic patterned indented grating, and an isotropic overcoat layer is further loaded on thereon. Thus, a polarization beam-splitting device is obtained which divides an incident light with orthogonal polarization directions into a 0-th light and diffracted lights. In this device, the above-mentioned birefringent film includes a high polymer birefringent film (for example, an organic drawn high polymer film).
Moreover, Japanese laid-open patent application No. 9-63111 discloses one example of a laser light source employing a polarization hologram device. In this art, in order to achieve a configuration in that a light-emitting device and a light-receiving device for signal detection are mounted in a common cap, a polarization hologram device is applied, and, a part of a light obtained from the hologram device is utilized as an output monitoring light.
However, according to a theory, such a type of a polarization hologram device has a maximum possible diffraction efficiency of as high as approximately 40%. Moreover, when it is applied in an optical pickup etc., a laser light once passing through the polarization hologram device is reflected by a disk-type recording medium, and, after that, it is diffracted by the polarization hologram device, the thus-obtained light being then applied to a light-receiving device for signal detection. Accordingly, the actually applicable substantial overall diffraction efficiency may not be high enough. Moreover, a variation in the diffraction efficiency may also occur not only due to a particular product but also due to some error in assembly of a polarization hologram device into an optical pickup, or so. By these factors, the actually applicable diffraction efficiency thereof may not be expected sufficiently high.